1. Field of the Invention
The present invention relates generally to a system and method for an analog to digital (A/D) converter, and in particular, to a system and method for managing a bit-depth requirement for an A/D converter in a GPS receiver intended for operation in a high jamming environment.
2. Description of the Relation Art
Global Positioning Systems (GPS) are widely utilized in both commercial and military applications. Whether determining the location of a cellular telephone having a GPS tracking systems, navigating a commercial airliner or military jet, or guiding ordinance to particular targets, the reliance on GPS to provide accurate positioning is increasing.
Along with the increased reliance upon the GPS comes a growing need to provide more accurate and reliable GPS positioning. GPS signals can be adversely affected by various factors including, for example, low signal strength, noise and GPS jamming devices. Each of these causes the GPS system to provide false and inaccurate positioning data in the particular applications. In turn, this inaccurate or false data can cause devastating effects.
Standard GPS techniques for treating jammed signals involve two stages. First, a front-end processing stage is utilized to excise as much of the jammer signal as possible from the raw input; this is commonly referred to as jam suppression processing. Second, extensive correlation and tracking operations are utilized to remove the remaining interference and noise from the GPS signal; this is commonly referred to as signal-detection processing. These operations are most effectively performed in the digital domain. As such, receivers are currently being developed in which the conversion from analog to digital operation occurs after preliminary filtering but precedes all jam suppression and signal detection operations.
FIG. 1 is a block diagram of a conventional GPS receiver. Shown in FIG. 1 is antenna ANT for receiving an input signal. The input signal is comprised of GPS signal, noise and jamming signal. Connected to antenna ANT is low noise amplifier (LNA) 101 for amplifying the input signal. Next is filtering/down-conversion module 102 for filtering the input signal and down converting the filtered signal. The filtered and down-converted signal is then processed by A/D converter 103. According to this conventional structure, the full in-band jamming signal is present at the A/D converter 103 and must be processed by the A/D converter 103, resulting in a sampled/quantized signal output from the A/D converter 103. The processing of the full in-band jamming signal along with the GPS signal in the A/D converter 103 results in a much weaker GPS signal in the sampled/quantized signal being available to subsequent jam suppression processing module 104 and a much weaker GPS signal in a jam-suppressed signal being available to subsequent signal detection processing module 105. The above system outputs position (P), velocity (V), orientation (T), pseudorange, and phase components.
The above-described conventional structure requires significant bit-depth (i.e. word length) for the A/D converter 103 to properly convert the analog signal to a digital signal. Typical requirements in the conventional design are a bit-depth of 16-17 bits to accommodate sufficient code processing at a 120 dB jamming-to-signal (J/S) power ratio. Such resolution must be achieved at sample rates of 20-40 MHz to capture the GPS precision (P(Y)) code. The combination of these requirements presents major challenges to A/D technology in linearity, speed, resolution, and power consumption.
An additional concern is that the above bit-depth specification depends directly on the strength of the expected jamming signal. That is, the required number of bits increases logarithmically with the square root of jamming power. Conventional systems designed to a given bit-depth have a hard limit of jamming power that can be accommodated, whatever the power, sophistication and adaptability of the processing that is utilized after the A/D conversion.
Particularly troubling jamming systems are those in which a highly correlated jamming signal is used to jam the GPS signal. One example is a Gaussian jammer having a bandwidth that is much narrower than the GPS signal itself. In such a Gaussian jammer, jammer signal levels from one sample to the next are highly correlated. Another example is a pure tone (e.g. sine wave) jammer. In both of these examples, the jamming signal does not resemble a typical noise-like signal. Conventional A/D conversion yields highly deterministic values. Since the GPS signal is so far below the system thermal noise, elimination of randomness from the A/D converter 103 output effectively eliminates the GPS signal itself, after which no amount of subsequent processing can recover the signal. These examples present situations that require processing at the 16-17 bit bit-depth at the 120 dB J/S power ratio, as this bit-depth is required to, at the minimum, reach the system noise floor, which in turn ensures randomness in the digitized output, and provides enough overall dynamic range so as not to clip the largest values in the input signal.
There is therefore a need to provide system and method for managing a bit-depth requirement for an A/D converter in a GPS receiver intended for operation in a high jamming environment.